These are likely the 1/8’’ ones. According to 1987 letters to Mr. Dmochowski, Beksiński’s favorite hardboard were thickness 3.2 mm (⅛ inch). The board edges should be cut to a perfect straight line. Not bent, no damage to the surface. Any grease stains would disqualify a particular sheet material. It was Beksiński’s legitimate concern that grease stains can penetrate, and prevent drying of oil paint and acrylic primer.

Here you can watch a home video with Beksiński sanding and priming the fiberboards.